Bearing for alternator and bearing for pulley

ABSTRACT

A bearing for alternators and a bearing for pulleys for reducing hydrogen embrittlement or cracking is used when the bearing is in an environment having a severe condition, such as, when hydrogen is generated from grease or the like. The bearing includes a rolling element, an inner ring and an outer ring, and at least one of the rolling element, where the inner and outer rings have a nitrogen enriched layer containing austenite crystal grains having a grain size number exceeding 10.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to bearings for alternators andbearings for pulleys, and particularly to such bearings less prone toflake-accompanied, hydrogen embrittlement or cracking in an ambient ofhydrogen.

2. Description of the Background Art

In bearings used for alternators or pulleys there is often observed aunique fracture accompanied with flakes at its damage starting point.The flake is dissimilar to a white etching constituent (WEC) introducedby rolling fatigue, a so called butterfly introduced in the vicinity ofa non-metal inclusion and the like. The present flake is characterizedby failing to have directivity relative to a direction of rolling. Abearing having a damage accompanied with flakes at a starting pointapparently shows an increased amount of hydrogen in the steel, and inthe flake there is a crack, which extends along a grain boundary deepinside. As such, such a damage as described above has been considered tobe attributed to hydrogen. Hereinafter, this flake-accompanied, uniquefracture will be referred to as hydrogen embrittlement or cracking.

Hydrogen embrittlement or cracking is caused possibly because as thebearding rotates, a chemically active fresh metal surface is producedand its catalysis effect decomposes a lubricant and generates hydrogen,which enters into the steel. Accordingly, as an approach againsthydrogen embrittlement or cracking, (a1) using a chemically lessdecomposable lubricant and (a2) blackening to minimize the exposure offresh metal surface have been considered effective.

For item (a1), a bearing is proposed, as follows: alkyldiphenylether oiland poly α olefin oil are blended at a ratio in weight of 20:80 to 80:20to prepare a base oil, which is blended with 5-40% by weight of anaromatic diurea compound or an aromatic urea-urethane compound as athickener, and furthermore a grease composite with a passivationoxidizer and organic sulfonate added thereto is enclosed in a rollerbearing to provide a grease enclosed bearing for an alternator, asdisclosed in Japanese Patent Laying-open No. 5-263091.

For item (2 b), a grease enclosed bearing is proposed that has a racewayhaving a rolling contact surface covered with an oxide film of 0.1 to2.5 μm in thickness, as disclosed in Japanese Patent Laying-open No.2-190615.

The above proposals, as confirmed in the field of the art, provideeffectively prolonged lives.

In recent years, however, bearings for alternators and those for pulleysare used in increasingly severe environments, and simply taking theabove approach is no longer sufficient. There is a demand for anapproach that can provide an increased life against hydrogenembrittlement or cracking.

SUMMARY OF THE INVENTION

The present invention contemplates a bearing for alternators and abearing for pulleys capable of reducing hydrogen embrittlement orcracking despite that the bearing is used in an environment under such asevere condition that hydrogen is generated from grease or the like andthe bearing experiences a load of a condition facilitating hydrogenembrittlement or cracking.

The present invention provides a bearing for an alternator that supportsa rotor shaft having one end with a pulley attached thereto and having abelt engaged therearound to transmit a driving force to the pulley toallow the rotor shaft to rotate a rotor. The bearing includes a rollingelement, an inner ring and an outer ring, and at least one of therolling element and the inner and outer rings has a nitrogen enrichedlayer and contains austenite crystal grains having a grain size numberexceeding 10. Here, the austenite grains refer to crystal grains ofaustenite which is phase-transformed during the heating process, and thetraces of grains remain after the austenite is transformed intomartensite through quenching cooling as described above.

At least one of the rolling element and inner and outer rings containingaustenite grains reduced in size to have a grain size number exceeding10 that is combined with a nitrogen enriched layer significantlyeffectively reduces hydrogen embrittlement or cracking. This is possiblyattributed to the following reason: in a single crystal grain a certaincrystal plane provides hydrogen embrittlement or cracking and the crackdoes not exactly extend to adjacent crystal grains. To crack adjacentcrystal grains it is necessary to store additional energy to do so.Storing energy for example in a hydrogen atom at a specific portionrequires a period of time. This should contribute to a reduced rate ofhydrogen embrittlement or cracking.

The austenite grain size number may be determined by the usual methoddefined by JIS, or determined according to the intercept method forexample with the average grain size corresponding to the above grainsize number. A smaller austenite grain size is desirable and anaustenite grain size number of 11 or greater is further desirable.Alternatively, the average grain size may be 6 μm or less. The austenitegrain size number may be achieved in the nitrogen enriched layer. Ingeneral, however, the condition of the fineness of austenite issatisfied in the body of steel located inside the nitrogen enrichedlayer.

The above-described austenite crystal grain may be any austenite crystalgrain having a grain boundary that can be observed after being subjectedto a process manifesting a grain boundary such as an etching process fora metal phase sample of the member of interest. The austenite crystalgrain is sometimes referred to as a prior austenite grain as describedearlier when it has a grain boundary just heated and ready to besubjected to low-temperature quenching. For measurement of the grainsize, the average of JIS-defined grain size numbers may be converted toobtain an average grain diameter, as has been described above, or theintercept method or the like may be used in which a straight line isplaced on a metal phase structure in an arbitrary direction to obtain anaverage length between points at which the straight line meets grainboundaries and the average length may be multiplied by a correctioncoefficient to represent a two- or three-dimensional length between thepoints.

The nitrogen enriched layer may be formed by a carbonitriding process,as will be described hereinafter. The nitrogen enriched layer may or maynot be enriched with carbon.

A fine austenite grain as described above is obtained by carbonitridingsteel for a bearing component at a temperature higher than an A1transformation temperature, then cooling the steel to a temperature ofless than the A1 transformation temperature, and then reheating thesteel to a range in temperature of no less than the A1 transformationtemperature and less than an A3 transformation temperature and alsolower than the temperature applied to the carbonitriding process, andquenching the steel. This thermal treatment is referred to as alow-temperature secondary quenching. This production method will bedescribed more specifically in an embodiment.

In the same application the present invention provides still anotherbearing for an alternator with at least one of the rolling element andthe inner and outer rings having a nitrogen enriched layer and providinga fracture stress value of no less than 2,650 MPa.

The present inventors have found that the above thermal treatment allowssteel having a carbonitrided layer to provide a conventionallyunachievable stress value of no less than 2,650 MPa. A rolling bearingof large strength can be obtained exhibiting excellent durabilityagainst a radial load and a moment load superimposed on each other whenthe bearing is applied for an alternator.

In the same application the present invention provides still anotherbearing for an alternator with at least one of the rolling element andthe inner and outer rings having a nitrogen enriched layer and ahydrogen content of no more than 0.5 ppm.

The above described thermal treatment (the low-temperature secondaryquenching) can provide a reduced ratio of hydrogen contained in a memberof steel before it is used for the alternator. This can increase theperiod of time required for hydrogen entering the steel to reach acritical value at which the steel cracks, and together with other, stillunclarified reason can render the steel less prone to hydrogenembrittlement or cracking.

Smaller amounts of hydrogen are desirable. However, reducing an amountof hydrogen to less than 0.3 ppm requires heating for a long period oftime, and austenite grains would have a coarse and large diameter andtoughness would rather be impaired. Accordingly, an amount of hydrogenfalling within a range of 0.3 to 0.5 ppm is desirably contained. Moredesirably, an amount of hydrogen falling within a range of 0.35 to 0.45ppm is contained.

In measuring the above hydrogen content, diffusible hydrogen is notmeasured and only the non-diffusible hydrogen released from the steel ata predetermined temperature or higher is measured. Diffusible hydrogenin a sample of small size is released from the sample to be scatteredeven at room temperature, and therefore the diffusible hydrogen is notmeasured. Non-diffusible hydrogen is trapped in any defect in the steeland only released from the sample at a predetermined heating temperatureor higher. Even if only the non-diffusible hydrogen is measured, thehydrogen content considerably varies depending on the method ofmeasurement. The above mentioned range of hydrogen content is determinedby thermal conductimetry. In addition, as detailed later, themeasurement may be taken by means of a LECO DH-103 hydrogen determinatoror like measuring device.

In the present bearing for the alternator at least one of the rollingelement and the inner and outer rings having the nitrogen enriched layermay have a surface layer containing. globular carbide having an arearatio of no less than 10%.

The surface layer containing globular carbide having an area ratio of noless than 10% allows the bearing for the alternator to be furthermoreresistant to hydrogen embrittlement or cracking.

The present invention provides a bearing for a pulley that is posedbetween a pulley having a belt engaged therearound and a rotative shaftpassing through the pulley. The bearing includes a rolling element, aninner ring and an outer ring, and at least one of the rolling elementand the inner and outer rings has a nitrogen enriched layer and containsaustenite crystal grains having a grain size number exceeding 10.

Pulleys used in automobiles are increasingly miniaturized and subjectedto large loads. As such, a rotary shaft increasingly tends to rotaterapidly, and the bearing is used in a condition in which hydrogen isgenerated for example from grease. A pulley with the above describedbearing for a pulley arranged therewith can be driven with a beltarranged such that its center as seen in width is offset relative to acenter of the bearing. As such, the above described bearing experiencesa radial load and in addition a moment load and the bearing for thepulley has a component periodically experiencing a tensile stresscomponent inducing generated hydrogen to enter into the steel.

At least one of the rolling element and inner and outer rings containingaustenite grains reduced in size to achieve a grain size numberexceeding 10 that is combined with a nitrogen enriched layer cansignificantly reduce hydrogen embrittlement or cracking not only in abearing for an alternator but also a bearing for a pulley. Furthermore,the effect of providing at least one of the rolling element and innerand outer rings with a fracture stress value of 2,650 MPa and the effectof providing it with a hydrogen content of no more than 0.5 ppm can beconsidered similarly as considered for the bearing for the alternator.

In the present bearing for the pulley at least one of the rollingelement and the inner and outer rings having the nitrogen enriched layermay have a surface layer containing globular carbide having an arearatio of no less than 10%.

The bearing for the pulley that has a surface layer containing globularcarbide having an area ratio of no less than 10% can further be enhancedin resistance to hydrogen embrittlement or cracking.

In the above described bearings for the alternator and pulley,respectively, at least one of the rolling element, the inner ring andthe outer ring can be formed of material of steel of SUJ2 as defined byJIS. By subjecting the bearing's member of material of steel SUJ2 to theabove-described, low-temperature secondary quenching, fine austenitegrains, a high fracture stress value and a low hydrogen content, asdescribed above, can be readily obtained.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows an alternator including a bearing for an alternator inaccordance with the present invention in an embodiment;

FIG. 2 is an enlarged view of the FIG. 1 bearing;

FIG. 3 shows a bearing for a pulley in accordance with the presentinvention;

FIG. 4 represents an exemplary thermal treatment applied to at least oneof a rolling elements, an inner ring and an outer ring of bearings foran alternator and a pulley, respectively, in accordance with the presentinvention in an embodiment;

FIG. 5 represents another exemplary thermal treatment applied to atleast one of a rolling elements, an inner ring and an outer ring ofbearings for an alternator and a pulley, respectively;

FIGS. 6A and 6B show a microstructure, particularly austenite grains, ofa bearing component, FIG. 6A showing a bearing component of the presentinvention and FIG. 6B showing a conventional bearing component;

FIGS. 7A and 7B diagrammatically show austenite grain correspondingrespectively to FIGS. 6A and 6B;

FIG. 8 shows a test piece for testing static pressure fracture-strength(measuring fracture stress);

FIG. 9A is a front view of a rolling fatigue life tester and FIG. 9B isa side view thereof;

FIG. 10 shows a test piece for testing static fracture toughness;

FIGS. 11A-11C represent processes in accordance with the presentinvention in examples V1 and V2 and a comparative example C1,respectively;

FIG. 12 shows a tester reproducing hydrogen embrittlement or cracking;and

FIGS. 13A and 13B show hydrogen embrittlement or cracking in aphotograph and a schematic representation, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe the presentinvention in embodiment. FIG. 1 shows an alternator 10. A bearing 1, 15for the alternator receives a shaft 11 inserted therethrough. The shafthas an end protruding outer than the bearing and having a pulley 13attached thereto. Pulley 13 has a groove 16 along which a transmissionbelt (not shown) is wound and thus engaged with.

Bearing 1 is located between a rotor 12 fixed to shaft 12 at a centerand pulley 13 and is referred to as a front bearing. The otheralternator 15 is located between rotor 12 and the other end and referredto as a rear bearing. The front and rear bearings are both attached to ahousing. The present invention may be directed to either the front orrear bearing, although it should be noted that front bearing 1 is moreliable to hydrogen embrittlement or cracking as it is located betweenpulley 13 and rotor 12 and experiences a large bending moment or othersimilar stress.

FIG. 2 is an enlarged view of the FIG. 1 front bearing 1. Front bearing1 is configured of an inner ring 5, an outer ring 7, a rolling element 3and a seal 8. Inner and outer rings 5 and 7 have rolling contactsurfaces 5 a and 7 a, respectively, together holding rolling element 3.The rolling element may be formed of steel or ceramic. Inner ring 5 ispress-fitted on shaft 11, and shaft 11 is rotatably supported.Furthermore, rolling element 3 is held by a cage (not shown) and betweeninner and outer rings 5 and 7 at opposite ends seal 8 is arranged toseal the bearing's interior. In the sealed interior there is a space inwhich grease is enclosed.

FIG. 3 shows a pulley 20 including a bearing 21 for the pulley inaccordance with the present invention in an embodiment. Pulley 20includes a main body 19 fitted on the bearing's outer ring 7 externally,and a portion 18 receiving a belt wound therearound. Bearing 21 isconfigured of inner ring 5, outer ring 7, rolling element 3 and seal 8.Inner and outer rings 5 and 7 have rolling contact surfaces 5 a and 7 a,respectively, together holding rolling element 3 therebetween. Main body19 externally fitted on outer ring 7 is rotatably supported.Furthermore, rolling element 3 is held by a cage (not shown) and betweeninner and outer rings 5 and 7 at opposite ends seal 8 is arranged toseal the bearing's interior. In the sealed interior there is a space inwhich grease is enclosed.

Furthermore in FIG. 3 inner ring 5 may be integrated with a shaft usedto attach pulley 20 to a counterpart member. More specifically, it mayinclude such a type that a shaft has an outer diameter with a groove.

In the above described bearing, at least one of outer ring 7, inner ring5 and rolling element 3 contains steel having a nitrogen enriched layerand contains austenite grains having a grain size number exceeding 10.Alternatively, at least one of outer ring 7, inner ring 5 and rollingelement 3 contains steel having a nitrogen enriched layer and alsoachieve a fracture stress value of no less than 2650 MPa. Alternatively,at least one of outer ring 7 inner ring 5 and rolling element 3 containssteel having a nitrogen enriched layer and a hydrogen content of no morethan 0.5 ppm.

The above described, at least one of the inner and outer rings and therolling element used at a high velocity and under a high surfacepressure which are enclosed by grease, hardly provides hydrogenembrittlement or cracking and can be increased in longevity.

These rolling bearings have at least one of their outer and inner ringsand rolling elements undergoing a thermal treatment includingcarbonitriding.

FIG. 4 shows a thermal treatment pattern according to which primaryquenching and secondary quenching are carried out, and FIG. 5 shows athermal treatment pattern according to which a material is cooled to atemperature lower than the A1 transformation temperature in a quenchingprocess and thereafter heated again to be finally quenched. Both areembodiments of the present invention. Referring to these figures, inprocess T1, carbon and nitrogen are diffused through a steel matrixwhile the carbon is sufficiently dissolved therein, and thereaftercooling is done to a temperature below the A1 transformationtemperature. Then, in process T2 shown in the drawings, heating is doneagain to a temperature lower than that in process T1 and thenoil-quenching is performed.

The above described thermal treatments both allow their carbonitridingprocesses to produce a nitrogen-enriched layer that is a “carbonitridedlayer.” Since the material for the carbonitriding process, the steel,has a high concentration of carbon, carbon in the atmosphere of thenormal carbonitriding process might not enter the surface of the steeleasily. For example, with steel having a high concentration of carbon(approximately 1% by weight), a carburized layer may have a higherconcentration of carbon than this value, or a carburized layer may beformed without having a higher concentration of carbon than this value.A concentration of nitrogen in normal steel, however, is typically aslow as approximately no more than 0.025 wt % at the maximum although itdepends on a concentration of Cr or the like. Therefore, a nitrogenenriched layer can apparently be formed regardless of the concentrationof carbon in source steel. It will be appreciated that the abovedescribed nitrogen enriched layer may also be enriched with carbon.

Compared with normal quenching, by which carbonitriding is done andimmediately thereafter quenching is done once, the above discussedthermal treatment can provide enhanced crack strength and reducedsecular dimensional variation rate while carbonitriding the surfacelayer. This thermal treatment can also produce a microstructure havingaustenite crystal grains of a grain size smaller than conventional byone half or more. A bearing component subjected to the above describedthermal treatment can have a long rolling fatigue life, an increasedanti-crack strength, and a reduced secular dimensional variation rate,and also exhibit significant resistance to hydrogen embrittlement orcracking.

FIG. 6A shows a grain of austenite of a bearing steel having beenheat-treated as shown in FIG. 4. For comparison, FIG. 6B shows a grainof austenite of a bearing steel which has undergone the conventionalthermal treatment. FIGS. 7A and 7B diagrammatically show the grains ofaustenite that are shown in FIGS. 6A and 6B. In the structures with thecrystal grains of austenite, the grain diameter of the conventionalaustenite is 10 which is a grain size number defined by JIS while thatof the present invention through the thermal treatment thereof is forexample 12 and thus fine grains are seen. Further, the average graindiameter in FIG. 6A is 5.6 μm measured by the intercept method.

The present invention in examples will now be described.

EXAMPLE 1

JIS-SUJ2 (1.0 wt % of C—0.25 wt % of Si—0.4 wt % of Mn—1.5 wt % of Cr)was used for Example 1 of the present invention. Samples shown in Table1 were each produced through the procedure described below.

JIS G4805 SUJ2 specifies the range of chemistry as follows:

C: 0.95-1.10 wt %, Si: 0.15-0.35 wt %, Mn: ≦0.50 wt %,

P: ≦0.025 wt %, S: ≦0.025 wt %, Cr: 1.30-1.60 wt %

TABLE 1 sample conventional normal carbonitrided quenched A B C D E Fsample sample secondary 780¹⁾ 800 815 830 850 870 — — quenchingtemperature (° C.) hydrogen amount — 0.37 0.40 0.38 0.42 0.40 0.72 0.38(ppm) crystal grain size — 12 11.5 11 10 10 10 10 (JIS) Charpy impact —6.65 6.40 6.30 6.20 6.30 5.33 6.70 value (J/cm²) fracture stress — 28402780 2650 2650 2700 2330 2770 (MPa) relative rolling — 5.4 4.2 3.5 2.92.8 3.1 1 fatigue life (L₁₀) ¹⁾evaluation failed due to insufficientquenching

Samples A-D Examples of the Present Invention

Carbonitriding was performed at 850° C. held for 150 minutes in anatmosphere of a mixture of RX gas and ammonia gas. Following the thermaltreatment pattern shown in FIG. 4, primary quenching was done from acarbonitriding temperature of 850° C., and secondary quenching wassubsequently done by heating to a temperature in a temperature rangefrom 780° C. to 830° C. lower than the carbonitriding temperature.Sample A with a secondary quenching temperature of 780° C. was nottested since quenching of sample A was insufficient.

Samples E and F Comparative Examples

These samples were carbonitrided through the same procedure as that ofsamples A-D of the present invention, and then secondary quenched at atemperature from 850° C. to 870° C. equal to or higher than thecarbonitriding temperature of 850° C.

Conventional Carbonitrided Sample: Comparative Example

Carbonitriding was performed at 850° C. held for 150 minutes in anatmosphere of a mixture of RX gas and ammonia gas. Quenching wassuccessively done from the carbonitriding temperature and no secondaryquenching was done.

Normal Quenched Sample: Comparative Example

Without carbonitriding, quenching was done by increasing the temperatureto 850° C. and no secondary quenching was done.

For the samples above, tests were conducted for (1) measuring the amountof hydrogen, (2) measuring crystal grain size, (3) Charpy impact test,(4) measuring fracture stress and (5) rolling fatigue test, by themethods described below.

I. Test Methods for Example 1

(1) Measurement of Hydrogen Amount

The amount of hydrogen was determined by means of a DH-103 hydrogendeterminator manufactured by LECO Corporation to analyze the amount ofnon-diffusible hydrogen in a steel. The amount of diffusible hydrogenwas not measured. Specifications of the LECO DH-103 hydrogendeterminator are as follows:

Analysis range: 0.01-50.00 ppm

Analysis precision: ±0.1 ppm or ±3% H (higher one)

Analysis sensitivity: 0.01 ppm

Detection method: thermal conductimetry

Sample weight size: 10 mg-35 g (max: 12 mm (diameter)×100 mm (length))

Furnace temperature range: 50° C.-1100° C.

Reagent: anhydron Mg(ClO₄)₂, Ascarite and NaOH

Carrier gas: nitrogen gas

Dosing gas: hydrogen gas

(Both gases have a purity of at least 99.99% and a pressure of 40 PSI(2.8 kgf/cm²).)

The procedure of the analysis is roughly described here. A sample wastaken by a dedicated sampler and the sample together with the samplerwas put into the hydrogen determiner. Diffusible hydrogen therein wasdirected by the nitrogen carrier gas to a thermal conductimetrydetector. The diffusible hydrogen was not measured in this example.Then, the sample was taken out of the sampler to be heated in aresistance heater and non-diffusible hydrogen was directed by thenitrogen carrier gas to the thermal conductimetry detector. The thermalconductivity was measured by the thermal conductimetry detector todetermine the amount of non-diffusible hydrogen.

(2) Measurement of Crystal Grain Size

The crystal grain size was measured according to the method of testingthe crystal grain size of austenite in a steel defined by JIS G 0551.

(3) Charpy Impact Test

A Charpy impact test was conducted according to the Charpy impact testmethod for a metal material defined by JIS Z 2242. A test piece usedhere was a U-notch test piece (JIS No. 3 test piece) defined by JIS Z2202.

(4) Measurement of Fracture Stress

FIG. 8 shows a test piece for a static-pressure fracture-strength test(for measuring fracture stress). A load was exerted in direction P inFIG. 8 and the load when the test piece was fractured was measured.Then, the measured load which was a fracture load was converted into astress by the following stress calculation formula for a curved beam. Itis noted that the test piece to be used is not limited to the one shownin FIG. 13 and may be any test piece having a different shape.

Suppose that a fiber stress on the convex surface of the test pieceshown in FIG. 8 is σ₁ and a fiber stress on the concave surface is σ₂,then, σ₁ and σ₂ are determined by the following formula (JSME MechanicalEngineer's Handbook, A4-strength of materials, A4-40). Here, N indicatesan axial force of a cross section including the axis of the annular testpiece, A indicates a cross-sectional area, e₁ indicates an outer radius,e₂ indicates an inner radius, and κ is a section modulus of the curbedbeam.σ₁=(N/A)+{M/(Aρ_(o))}[1+e ₁/{κ(ρ_(o) +e ₁)}]σ₂=(N/A)+{M/(Aρ_(o))}[1−e ₂/{κ(ρ_(o) −e ₂)}]κ=−(1/A)∫A{η/(ρ_(o)+η)}dA

(5) Rolling Fatigue Test

Test conditions for a rolling fatigue life test are shown in Table 2.FIGS. 9A and 9B schematically show a rolling fatigue life tester, FIG.9A being a cross-sectional view and FIG. 9B being a side view thereof.Referring to FIGS. 9A and 9B, a test piece 61 undergoing the rollingfatigue life test was driven by a driving roll 51 to rotate while beingin contact with balls 53. Balls 53 were (¾)″ balls guided by a guidingroll 52 to roll. Balls 53 exerted a high surface pressure on test piece61 while test piece 61 also exerted a high surface pressure on balls 53.

II. Results of Tests for Example 1

(1) Amount of Hydrogen

As shown in table 1, the conventional carbonitrided sample without beingadditionally processed has a considerably large hydrogen amount of 0.72ppm. A reason therefor is considered that ammonia (NH₃) contained in theatmosphere in the carbonitriding process is decomposed and then hydrogenenters the steel. On the other hand, the hydrogen amount of samples B-Dis reduced to 0.37-0.40 ppm and thus almost a half of that of theconventional sample. This amount of hydrogen is substantially equal tothat of the normal quenched sample.

The above-mentioned reduction of the hydrogen amount can lessen thedegree of embrittlement of the steel that is due to hydrogen in thesolid solution. In other words, by the reduction of the hydrogen amount,the Charpy impact value of samples B-D of the present invention isremarkably improved.

(2) Crystal Grain Size

Regarding the crystal grain size, samples that are secondary quenched ata temperature lower than the quenching temperature in the carbonitridingprocess (primary quenching), namely samples B-D have austenite grainswhich are remarkably made fine, i.e., crystal grain size number is11-12. Samples E and F as well as the conventional carbonitrided sampleand the normal quenched sample have austenite grains with the crystalgrain size number of 10, which means that the crystal grain size ofsamples E and F is greater than that of samples B-D of the presentinvention.

(3) Charpy Impact Test

Table 9 shows that the Charpy impact value of the conventionalcarbonitrided sample is 5.33 J/cm² while that of samples B-D of thepresent invention is higher, ranging from 6.30 to 6.65 J/cm². It is alsoseen from this that a lower secondary quenching temperature leads to ahigher Charpy impact value. The normal quenched sample has a high Charpyimpact value of 6.70 J/cm².

(4) Measurement of Fracture Stress

The fracture stress corresponds to anti-crack strength. It is seen fromTable 1 that the fracture stress of the conventional carbonitridedsample is 2330 MPa. On the other hand, the fracture stress of samplesB-D is improved to 2650-2840 MPa. The normal quenched sample has afracture stress of 2770 MPa which is in the range of the. fracturestress of samples B-F. It is considered that the reduction in hydrogencontent greatly contributes to the improved anti-crack strength ofsamples B-D as well as the reduction in size of austenite crystalgrains.

(5) Rolling Fatigue Test

According to Table 1, the normal quenched sample has the shortestrolling fatigue life (L10) due to the absence of carbonitrided layer inthe surface layer. In contrast, the rolling fatigue life of theconventional carbonitrided sample is 3.1 times as long as that of thenormal quenched sample. The rolling fatigue life of samples B-D isremarkably improved as compared with the conventional carbonitridedsample. Samples E and F of the present invention have the rollingfatigue life almost equal to that of the conventional carbonitridedsample.

In summary, samples B-D of the present invention have the lower hydrogencontent, finer austenite crystal grains with the crystal grain sizenumber of at least 11, and improved Charpy impact value, anti-crackstrength and rolling fatigue life.

EXAMPLE 2

Example 2 of the present invention is now described. On the followingsamples X, Y and Z, a series of tests was conducted. A material to beheat-treated that was employed commonly to samples X-Z was JIS-SUJ2 (1.0wt % of C—0.25 wt % of Si—0.4 wt % of Mn—1.5 wt % of Cr). Samples X-Zwere each processed through the following procedure.

Sample X—comparative example: normal quenching only (withoutcarbonitriding)

Sample Y—comparative example: quenching directly after carbonitriding(conventional carbonitriding and quenching) Carbonitriding was conductedat 845° C. held for 150 minutes. The atmosphere in the carbonitridingprocess was a mixture of RX gas and ammonia gas.

Sample Z—example of the present invention: A bearing material wasprocessed following the thermal treatment pattern shown in FIG. 4.Carbonitriding was conducted at 845° C. held for 150 minutes. Theatmosphere in the carbonitriding process was a mixture of RX gas andammonia gas. Final quenching temperature was 800° C.

(1) Rolling Fatigue Life

Test conditions and the test device for the rolling fatigue life testare as shown in Table 2 and FIGS. 9A and 9B. Results of the rollingfatigue life test are shown in Table 3.

TABLE 2 test piece φ12 × L22 cylindrical test piece number of testedpieces 10 counterpart steel ball ¾″ (19.05 mm) contact surface pressure5.88 GPa load speed 46240 cpm lubricating oil turbine VG68 - forcedcirculation lubrication

TABLE 3 life (load count) sample L₁₀ (×10⁴ times) L₁₀ (×10⁴ times)relative L₁₀ X 8017 18648 1.0 Y 24656 33974 3.1 Z 43244 69031 5.4

According to Table 3, sample Y which is a comparative example has arolling fatigue life (L10 life: one out of ten test pieces beingdamaged) that is 3.1 times as long as that of sample X which is also acomparative example and undergoes normal quenching only, and thus it isseen that the effect of extending the life is obtained through thecarbonitriding process. In contrast, sample Z of the present inventionhas a longer life which is 1.74 times as long as that of sample Y and5.4 times as long as that of sample X. It is considered that thisimprovement is obtained mainly from the fine microstructure.

(2) Charpy Impact Test

A Charpy impact test was conducted by using a U-notch test piece definedby JIS Z 2242 mentioned above. Test results are shown in Table 4.

TABLE 4 Charpy impact relative impact sample value (J/cm²) value X 6.71.0 Y 5.3 0.8 Z 6.7 1.0

Sample Y (comparative example) having undergone carbonitriding has aCharpy impact value which is not larger than that of sample X(comparative example) having undergone normal quenching, while sample Zhas a Charpy impact value equivalent to that of sample X.

(3) Static Fracture Toughness Test

FIG. 10 shows a test piece for a static fracture toughness test. In thenotch of the test piece, a pre-crack of approximately 1 mm was made,thereafter a static load by three-point bending was added, and then afracture load P was determined. Using the following formula (I), afracture toughness value (KIc value) was calculated. Results of the testare shown in Table 5.KIc=(PL√a/BW ²){5.8−9.2(a/W)+43.6(a/W)²−75.3(a/W)³+77.5(a/W)⁴}  (I)

TABLE 5 sample number tested K₁C (MPa√m) relative K₁C X 3 16.3 1.0 Y 316.1 1.0 Z 3 18.9 1.2

As the pre-crack has a depth greater than the depth of the carbonitridedlayer and thus the same results are obtained for samples X and Y(comparative examples), while the result for sample Z (example of thepresent invention) is approximately 1.2 times as high as that of thecomparative examples.

(4) Static-Pressure Fracture-Strength Test (Measurement of FractureStress)

A static-pressure fracture-strength test piece as shown in FIG. 8described above was used. A load was exerted in direction P in FIG. 13to conduct a static-pressure fracture-strength test. Test results areshown in Table 6.

TABLE 6 number static fracture relative static sample tested strength(kgf) fracture strength X 3 4200 1.00 Y 3 3500 0.84 Z 3 4300 1.03

Sample Y having been carbonitrided has a strength somewhat smaller thanthat of sample X having undergone normal quenching, while sample Z ofthe present invention has an improved static-pressure fracture-strengthcompared with sample Y and is thus equivalent to that of sample X.

(5) Rate of Secular Dimensional Change

Table 7 shows the rate of secular dimensional change measured under theconditions of 130° C. (holding temperature) and 500 hours (holdingtime), together with the surface hardness and the amount of retainedaustenite (0.1 mm depth).

TABLE 7 rate of surface dimensional relative rate of number hardnessretained change dimensional sample tested (HRC) Y^((%)) (×10⁻⁵) change*⁾X 3 62.5  9.0 18 1.0 Y 3 63.6 28.0 35 1.9 Z 3 60.0 11.3 22 1.2 *⁾smalleris superior

As compared with the rate of dimensional change of sample Y having alarge amount of retained austenite, sample Z of the present inventionhas the smaller rate of dimensional change due to the lower amount ofretained austenite which is one-half or less.

(6) Life Test under Contaminated Lubricant Condition

Ball bearing 6206 was used to evaluate the rolling fatigue life under acontaminated lubricant condition having a predetermined amount of normalcontaminants mixed therein. Test conditions are shown in Table 8 andtest results are shown in Table 9.

TABLE 8 load Fr = 6.86 kN contact surface pressure Pmax = 3.2 Gparotational speed 2000 rpm lubricant turbine 56 - oil bath lubricationamount of contaminant 0.4 g/1000 cc contaminant grain size: 100-180 μm,hardness: Hv800

TABLE 9 sample L₁₀ life_(h) relative L₁₀ X 20.0 1.0 Y 50.2 2.5 Z 74 3.7

Sample Y having undergone conventional carbonitriding has a lifetimewhich is approximately 2.5 times as long as that of sample X, and sampleZ of the present invention has a lifetime which is approximately 3.7times as long as that of sample X. While sample Z of the presentinvention has a smaller amount of retained austenite than that of sampleY of the comparative example, sample Z has a long lifetime substantiallyequivalent to that of sample Y because of influences of enteringnitrogen and the fine microstructure.

It is accordingly seen from the above-discussed results that, sample Zof the present invention, namely a bearing component produced by thethermal treatment method of the present invention can simultaneouslyachieve three goals: extension of the rolling fatigue life that has beendifficult to achieve by the conventional carbonitriding, improvement incrack strength and reduction of the rate of secular dimensional change.

EXAMPLE 3

SUJ2 having undergone two types of processes as shown in FIGS. 11A and11B was prepared as a material of steel as an example of the presentinvention minimizing hydrogen embrittlement or cracking, as describedabove.

FIG. 11A represents the present invention in an example V1 providing aprocess. More specifically, steel is carbonitrided at a high carbonpotential (CP) value of 1.3-1.6 and then from that temperature the steelis oil-quenched. The steel carbonitrided in an ambient with such a highCP value can have a surface layer containing globular or spherodizedcarbide having an area ratio of no less than 10%. The steel is thentempered at 180° C.

FIG. 11B represents the present invention in an example V2 providing aprocess. More specifically, steel is initially carbonitrided andquenched, as shown in comparative example C1 (see FIG. 11C), and thenheated to T1° C. (845° C.)−α° C., a temperature lower than an initialquenching temperature by α° C., and oil-quenched. The temperature(T1-α)° C. is no lower than the A1 transformation temperature and lowerthan the temperature of the preceding, carbonitriding and quenchingstep. As shown in the figure, quenching the steel starts at thetemperature of the carbonitriding step, and the quenching temperatureand the carbonitriding temperature are the same. In the presentdescription, a process performed to heat a medium to (T1−α)° C. andquench it will be referred to as low-temperature quenching. After themedium is low-temperature quenched, it is tempered at 180° C.

Furthermore, as a material of steel for a comparative example, fourmaterials of steel, i.e., SUJ2 processed as shown in FIG. 11C, SUJ2normally quenched, SUJ2 blackened, and 13% Cr steel are used forevaluation for comparison. Table 10 is a list of materials of steelevaluated.

TABLE 10 List of Subjects to be Evaluated Type of Steel Process NoteInvention in JIS-SUJ2 See FIG. 1 Large amount of Ex. V1 globular carbideInvention in JIS-SUJ2 See FIG. 1 Fine crystal grains + Ex. V2 largeamount of globular carbide Comparative JIS-SUJ2 See FIG. 1 Nitridedlayer Ex. C1 Comparative JIS-SUJ2 Normally quenched — Ex. C2 ComparativeJIS-SUJ2 Normally quenched + Oxide film Ex. C3 blackened Comparative 13%Cr Normally quenched Passive thin Ex. C4 steel coating (oxide film)

They were evaluated for hydrogen embrittlement or cracking in a testusing a radio load tester as shown in FIG. 12. A bearing 71 a, 71 b tobe tested was incorporated into two, opposite sides of a main shaftsupported by a bearing 73 attached to a center and rotated via a pulleyas it is driven by a servo motor. The bearing to be tested receives aradial, biased load and thus flexes as bearing 73 receives a radialload. Bearing 71 a, 71 b to be tested is controlled in temperature by acartridge heater 72 a, 72 b attached to a bearing housing. Thisevaluation test was conducted with bearing 71 a, 71 b in the form of aradial needle bearing having an outer diameter of 32 mm, an innerdiameter of 24 mm, and a length of 19.8 mm. The test was conducted underconditions as shown in Table 11.

TABLE 11 Test Conditions Contact surface pressure (GPa) 2.61/2.26 Innerring-side/outer ring-side Rate of rotation (rpm) Rapid acceleration anddeceleration of 500

→ 3,000 Testing temperature (° C.) 100 Lubricant Water-glycol based,incombustible, working oil Condition of lubrication Oil bath

The Table 11 test conditions with rapid acceleration and decelerationallow a sufficient reproduction of hydrogen embrittlement or cracking.Table 12 shows a result of the test conducted under these conditions.

TABLE 12 Test Result L₁₀ L₁₀ No. Longevity Longevity Type of SteelProcess of N (h) ratio Invention in Ex. V1 JIS-SUJ2 See FIG. 1 10 54.34.6 Invention in Ex. V2 JIS-SUJ2 See FIG. 1 10 >122.2*¹ >10.3Comparative Ex. C1 JIS-SUJ2 See FIG. 1 10 32.6 2.7 Comparative Ex. C2JIS-SUJ2 Normally quenched 10 11.9 1.0 Comparative Ex. C3 JIS-SUJ2Normally quenched + 6 62.5*² 5.3 blackened Comparative Ex. C4 13% Crsteel Normally quenched 6 70.6*² 5.9 *¹Indicated the longevity as therewas only a single crack. *²There was no flake observed thatcharacterizes hydrogen embrittlement or cracking

FIGS. 13A and 13B show a cross section including hydrogen embrittlementor cracking introduced in comparative example C1. FIGS. 13A and 13B showa fracture in a photograph and a schematic representation, respectively.It can be understood that under the above test conditions, hydrogenembrittlement or cracking is induced.

It can be understood from the Table 12 result that the examples of thepresent invention are longer in longevity than the comparative examplescorresponding to conventional products. In particular, the presentinvention in example V2 is further longer in longevity than eithercomparative example C2, a blackened material, and comparative exampleC3, 13% Cr steel blackened, both enhanced in resistance to hydrogenembrittlement or cracking.

To examine why the present invention in example V2 provides aparticularly long life, the present invention in examples V1 and V2 andcomparative examples C1 and C2 corresponding to conventional productshad their respective materials of steel measured at a surface layer toobtain their globular carbide area ratios and austenite grain numbers asdefined by JIS. Table 13 shows a result thereof.

TABLE 13 Globular Carbide Area Ratio and Average Austenite Grain SizeType Area ratio JIS defined of Steel Process (%) grain size Invention inEx. V1 JIS-SUJ2 See FIG. 1 12.9 10.1 Invention in Ex. V2 JIS-SUJ2 SeeFIG. 1 13.2 12.7 Comparative Ex. C1 JIS-SUJ2 See FIG. 1 7.6 10Comparative Ex. C2 JIS-SUJ2 Normally 7.9 10.2 quenched

The present invention in example V1 contains globular carbide having alarger area ratio than comparative example C1 and a JIS defined grainsize which does not have a significant difference than the comparativeexamples. The present invention in example V2 contains globular carbidehaving a larger area ratio than the comparative examples and a JISdefined grain size smaller than the comparative examples.

Comparative example C2 provides a longer life than comparative exampleC1, and the present invention in example V1 provides a further longerlife than comparative example C2 and the present invention in example V2provides a further longer life than the present invention in example V1.This reveals that hydrogen embrittlement or cracking can effectively bereduced by the following three factors:

(1) Presence of a nitrogen enriched layer;

(2) Globular carbide having an area ratio of no less than 10%; and

(3) Austenite grain having a JIS defined grain size number exceeding 10.

The present invention in example V2, satisfying all of the above threefactors, achieves a significantly long life. Simply satisfying item (1)alone also effectively reduces hydrogen embrittlement or cracking,although it does not provide prolonged life effectively, andaccordingly, satisfying at least items (1) and (2), as in the presentinvention in example V1, can provide a rolling bearing less prone tohydrogen embrittlement or cracking in an ambient of hydrogen.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A bearing for an alternator, supporting a rotor shaft having one endwith a pulley attached thereto and having a belt engaged therearound totransmit a driving force to said pulley to allow said rotor shaft torotate a rotor, the bearing having a rolling element, an inner ring andan outer ring, at least one of said rolling element and said inner andouter rings having a nitrogen enriched layer and containing austenitecrystal grains having a grain size number exceeding 10, wherein said atleast one of said rolling element and said inner and outer rings havingsaid nitrogen enriched layer has a surface layer containing globularcarbide having an area ratio of no less than 10%, and said at least oneof said rolling element and said inner and outer rings is formed ofsteel of SUJ2 as defined by JIS.
 2. A bearing posed between a pulleyhaving a belt engaged therearound and a rotative shaft passing throughsaid pulley, the bearing comprising a rolling element, an inner ring andan outer ring, at least one of said rolling element and said inner andouter rings having a nitrogen enriched layer and containing austenitecrystal grains having a grain size number exceeding 10, wherein said atleast one of said rolling element and said inner and outer rings havingsaid nitrogen enriched layer has a surface layer containing globularcarbide having an area ratio of no less than 10%, and said at least oneof said rolling element and said inner and outer rings is formed ofsteel of SUJ2 as defined by JIS.